Magnetotactic bacteria containing intracellular membrane-bound nanocrystals of magnetite or greigite (Bazylinski, et al. 2004; Flies, et al. 2005). The bacteria may also have intracellular inclusions (Keim, et al. 2005). Magnetosome formation in magnetotactic bacteria provides a novel magnetic nanomaterial that is generated by a mineralization process that controls the chemical composition, morphology and size of the magnetic mineral (Lang and Schüler, 2006). The aim of this work was to study magnetotactic cocci present in the estuary environment in Colombia, through transmission electron microscopy. Samples of sediment and water were collected at Cispata Beach, and these were incubated in the laboratory under low-light conditions at room temperature for several weeks in order to isolate magnetotactic bacteria using a magnetic field. Cells were studied using Morgagini and TECNAI G2 20 D345 (FEI) transmission electron microscopes operating at 80 kV and 200 kV respective. Magnetotactic coccus ranged from 0.5 to 1 μm in diameter. Each bacterium has four magnetosome chains with elongated prismatic crystals from 60 to 190 nm in length. Energy dispersive X-ray spectrometry (EDX) microanalysis of a magnetosome showed mainly iron and oxygen peaks probably of magnetite crystals. EDX microanalysis in other globular inclusions in the bacteria, shows mainly sulfur.